A semiconductor lithography machine includes a moveable stage whose position must be known with great certainty. Such measurements are conventionally provided by a system of interferometers, each of which illuminates the stage with a measurement beam and a stationary target (for example, the projection optics column of the semiconductor lithography machine) with a reference beam. These interferometers combine the returning measurement beam and reference beam and observe the resulting interference between them. This interference is indicative of a difference in the path lengths traversed by the two beams, and hence the movement of the stage.
In many semiconductor lithography machines, the stationary target is located at some distance from the stage. As a result, the measurement beam and the reference beam are separated by a considerable distance. Conventional interferometers accommodate this distance by providing a monolithic structure that is long enough so that the reference beam and the measurement beam can exit the interferometer parallel to each other. Known monolithic structures for such interferometers are prone to introducing errors resulting from thermal expansion and beam shear.